Lamport's distributed mutual exclusion algorithm
Lamport's Distributed Mutual Exclusion Algorithm is a contention-based algorithm for mutual exclusion on a distributed system.
Algorithm
[edit]Nodal properties
[edit]- Every process maintains a queue of pending requests for entering critical section in order. The queues are ordered by virtual time stamps derived from Lamport timestamps.[1]
Algorithm
[edit]Requesting process
- Pushing its request in its own queue (ordered by time stamps)
- Sending a request to every node.
- Waiting for replies from all other nodes.
- If own request is at the head of its queue and all replies have been received, enter critical section.
- Upon exiting the critical section, remove its request from the queue and send a release message to every process.
Other processes
- After receiving a request, pushing the request in its own request queue (ordered by time stamps) and reply with a time stamp.
- After receiving release message, remove the corresponding request from its own request queue.
Message complexity
[edit]This algorithm creates 3(N − 1) messages per request, or (N − 1) messages and 2 broadcasts. 3(N − 1) messages per request includes:
- (N − 1) total number of requests
- (N − 1) total number of replies
- (N − 1) total number of releases
Drawbacks
[edit]This algorithm has several disadvantages. They are:
- It is very unreliable as failure of any one of the processes will halt progress.
- It has a high message complexity of 3(N − 1) messages per entry/exit into the critical section.
See also
[edit]- Ricart–Agrawala algorithm (an improvement over Lamport's algorithm)
- Lamport's bakery algorithm
- Raymond's algorithm
- Maekawa's algorithm
- Suzuki–Kasami algorithm
- Naimi–Trehel algorithm
References
[edit]- ^ Kshemkalyani, A., & Singhal, M. Chapter 9: Distributed Mutual Exclusion Algorithms. Distributed Computing: Principles, Algorithms, and Systems (Page 10 of 93). Cambridge University Press.
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